Earth-boring tools are commonly used for forming (e.g., drilling and reaming) wellbores in earth formations. Earth-boring tools include, for example, rotary drill bits, coring bits, eccentric bits, bicenter bits, reamers, underreamers, and mills.
Different types of earth-boring rotary drill bits are known in the art including, for example, fixed-cutter earth-boring rotary drill bits (also referred to as “drag bits”), roller-cone earth-boring rotary drill bits (also referred to as “rock bits”), superabrasive-impregnated bits, and hybrid bits (which may include, for example, both fixed-cutters and rolling cutters). Fixed-cutter bits include a plurality of cutting elements that are fixedly attached to a bit body of the drill bit. Roller-cone earth-boring bits may include a plurality of cutting elements mounted to one or more cones thereof.
The drill bit is coupled, either directly or indirectly, to an end of what is referred to in the art as a “drill string,” which comprises a series of elongated tubular segments connected end-to-end that extends into the wellbore from the surface of the formation. Often, various tools and components, including the drill bit, may be coupled together at the distal end of the drill string at the bottom of the wellbore being drilled. This assembly of tools and components is referred to in the art as a “bottom hole assembly” (BHA).
The drill bit may be rotated within the wellbore by rotating the drill string from the surface of the formation, or the drill bit may be rotated by coupling the drill bit to a downhole motor, which is also coupled to the drill string and disposed proximate the bottom of the wellbore. The downhole motor may comprise, for example, a hydraulic Moineau-type motor having a shaft, to which the drill bit is attached, that may be caused to rotate by pumping fluid (e.g., drilling mud or fluid) from the surface of the formation down through the center of the drill string, through the hydraulic motor, out from nozzles in the drill bit, and back up to the surface of the formation through the annular space between the outer surface of the drill string and the exposed surface of the formation within the wellbore.
The cutting elements used in earth-boring tools often include polycrystalline diamond compact (often referred to as “PDC”) cutting elements, which are cutting elements that include a polycrystalline diamond (PCD) material. Such polycrystalline diamond compact cutting elements are formed by sintering and bonding together relatively small diamond grains or crystals under conditions of high pressure and high temperature, typically in the presence of a metal solvent catalyst (such as cobalt, iron, nickel, or alloys or mixtures thereof) to form a layer or “table” of polycrystalline diamond material on a cutting element substrate. These processes are often referred to as high-pressure, high-temperature (of “HPHT”) processes. The metal solvent catalyst material may be partially dispersed within and between the compacted diamond grains prior to HPHT sintering or during sintering processes to promote diamond-tip-diamond bonding, and to harden and strengthen the compacted diamond powder table.
Upon formation of a diamond table using the HPHT process, a fraction of the metal solvent catalyst material may remain in interstitial spaces between the grains of diamond in the resulting polycrystalline diamond table. The presence of the metal solvent catalyst material in the diamond table may contribute to thermal damage therein when the cutting element is heated by friction during use.
To overcome such problems, so called “thermally stable” polycrystalline diamond compacts (which are also known as thermally stable products, or “TSPs”) have been developed. Such a thermally stable polycrystalline diamond compact may be formed by leaching the metal solvent catalyst material (e.g., cobalt) out from interstitial spaces between the inter-bonded diamond crystals in the diamond table using, for example, an acid or combination of acids (e.g., aqua regia). A substantial amount of the metal solvent catalyst material may be removed from the diamond table, or metal solvent catalyst material may be removed from only a portion thereof. Thermally stable polycrystalline diamond compacts in which substantially all metal solvent catalyst material has been leached out from the diamond table have been reported to be thermally stable up to temperatures of about twelve hundred degrees Celsius (1,200° C.). However, responsive to exposure to temperatures exceeding such temperatures, the polycrystalline diamond compact may degrade (e.g., graphitize).